In the past, various types of prime movers, such as dynamoelectric machines or electric motors for instance, were provided with a winding circuit in a stationary assembly, such as a stator or the like, having parallel connected main or run winding means and an auxiliary or start winding means. During a starting mode of these past types of prime movers, the winding circuit thereof was initially connected across a power source or line by suitable operator operated switching means to effect the generally conjoint excitation of the main winding means and the start winding means at least until a rotatable assembly magnetically associated with the main winding means and the auxiliary winding means of the stationary assembly was energized generally to a preselected speed. Of course, various prior art mechanisms or devices were utilized to render the auxiliary winding means ineffective in the winding circuit generally when the preselected speed of the prime mover, i.e., the rotatable assembly thereof, was attained.
Some of these prior art devices for controlling the excitation of the auxiliary winding means were mechanical speed responsive devices. For instance, when the rotatable assembly attained the preselected speed, a centrifugal mechanism carried on the rotatable assembly was actuated to move an associated collar generally axially on the rotatable assembly from an at-rest position toward another position, and upon such axial movement to its another position, the collar drove linkage means for operating a switch which effected the de-excitation of the auxiliary winding means by disconnecting it from the power source. Thus, with the auxiliary winding means rendered ineffective in the winding circuit, the prime mover then was energized generally at the preselected speed thereof in response to the continued excitation of the main winding means. Of course, at least one of the disadvantageous or undesirable features of the aforementioned centrifugal mechanism and associated linkage means is believed to be those well-known limitations attributable to most all mechanical devices, such as tolerance problems, wear problems, noise problems and, in some instances reliability problems.
In other types of these prior art prime movers, a current relay was employed for controlling the excitation of the auxiliary winding means. For instance, a coil of the current relay was serially connected in the winding circuit with the main winding means thereof, and contact means of the current relay was serially connected in the winding circuit with the auxiliary winding means thereof. When the prime mover was energized across the line, as previously mentioned, the coil of the current relay and the main winding means were excited, and the excitation of the relay coil effected the magnetic closure of the contact means of the relay so as to effect the excitation of the auxiliary winding means; therefore; in this manner, the main winding means and the auxiliary winding means were conjointly excited to energize the prime mover to generally its preselected speed during a starting mode thereof. Of course, when the prime mover was so energized generally to its preselected speed, the current draw of the main winding means was appreciably reduced, and such reduction of current caused the relay coil to weaken its magnetic affect thereby to permit the contact means of the current relay to return to their at-rest or open position. Thus, with the relay contacts so opened to interrupt current flow to the auxiliary winding means, the auxiliary winding means were rendered ineffective generally at the time the prime mover attains its preselected speed, and the prime mover remained energized generally at the preselected speed thereof during its running mode in response to the continued excitation of the main winding means. At least one of the disadvantageous or undesirable features of such past prime movers utilizing a current relay for controlling excitation of the auxiliary winding means is believed to be that the contact means of the current relay had a tendency to weld in response to the generally large current draw by the auxiliary windings during the starting operation of the prime mover. Another disadvantageous or undesirable feature is believed to be that the current relay generated radio and television interference at the instant of make and break due to arcing of the contact means.
In the event a capacitor was utilized in series circuit relation with the auxiliary winding of the aforementioned prime mover utilizing the current relay, the starting torque of the such prime mover may be increased wherein the capacitor may limit the amount of current in the auxiliary winding to a safe value while electrically shifting its phase. Again, however, the aforementioned disadvantage or undesirable feature is believed to be that the allowable magnitude of the current drawn by the auxiliary winding is limited by the relay contact means to resist welding which may occur when the capacitor is discharged directly through the relay contact means during the starting operation of the prime mover. For instance, if the relay contact means make, break and remake during the starting operation of the prime mover, the capacitor may be charged to a peak voltage value, disconnected, and then reconnected one hundred eighty electrical degrees (180.degree.) later to a peak opposite polarity line. Thus, the resultant discharge is at double peak value. It is also believed that the use of a relatively large capacitor, for maximum starting torque of the aforementioned prime mover utilizing a current relay, may also be impracticable generally for the same reasons as discussed above.
Another one of the prior art devices for controlling the excitation of the auxiliary winding means was a positive temperature coefficient resistor (PTCR) which was connected in series with the auxiliary winding of the prime mover. When the prime mover was energized across the line, the main winding means and auxiliary winding means were initially conjointly excited since the PTCR exhibited little initial resistance to current flow to the auxiliary winding means. The PTCR acts generally as a low value resistor until its internal temperature reaches its "Curie" point or an anomaly temperature at which time its resistance increases abruptly along with an increase of its temperature. Therefore, the temperature of the PTCR was increased in response to current flow therethrough to the auxiliary winding means during the starting operation of the prime mover, and generally at the time the prime mover attained its preselected speed, the resistance of the PTCR abruptly increased to a value appreciably limiting current flow therethrough. Thus, the auxiliary winding means was, in effect, rendered generally ineffective in the winding circuit generally at the time the prime mover attained its preselected speed, and the prime mover remained energized generally at the preselected speed thereof in response to the continued excitation of the main winding means. Of course, so long as the PTCR is energized, it will remain "hot" exhibiting a high resistance to current flow, and it is believed that, under certain conditions, this may be a disadvantageous or undesirable feature. For instance, if for some reason the prime mover had not been energized to the preselected speed so that the torque of the main winding means alone would sustain such preselected speed, then the prime mover would stall to zero speed and remain there since the PTCR would, in effect, block current flow to the auxiliary winding means. If an overload protection system of the prime mover did not alleviate the aforementioned stalled condition or locked rotor condition of the prime mover or if such overload protection system did not reset itself before the PTCR had time to cool, this stalled condition of the prime mover would be perpetuated.
In the event the winding circuit of the prime mover was provided with run and/or start capacitors so that the PTCR was subjected to continuous voltage stress, it is believed that at least one disadvantageous or undesirable feature of this arrangement is that the PTCR may be subjected to voltage stress which may exceed the oversurface break down characteristic of the PTCR; therefore, as a result of this situation, it may have been necessary to provide the PTCR with a rather complicated and expensive external insulation system to inhibit such oversurface break down characteristics which, of course, is also believed to be a disadvantageous or undesirable feature.
In addition to the foregoing, if "heat sinks", such as metallic fins or other metal masses or the like, were employed in conjunction with the PTCR to hasten its recovery after it has been deenergized, it is believed that the PTCR was subjected to thermal stressing having a capacity to physically deteriorate or destroy the PTCR which is, of course, also believed to be a disadvantageous or undesirable feature.